Method for milling casting moulds

ABSTRACT

The invention relates to a method for producing heat-resistant casting molds from molding sand containing binder, particularly for producing an inner contour of casting molds for prototypes, large volumes and deep grooves often being produced especially in the machining of molds for rapid production of prototypes from casting materials. To increase efficiency, in a first step blocks of molding material are produced whose dimensions correspond to a mold cavity depth typically of 300 mm to 400 mm. The inner contour of the mold is then produced oversized with spacing close to the contour of the inner wall of the mold cavity by means of a roughing tool ( 4 ) that has an effective cutting diameter of 12 mm to 35 mm. The mold cavity is then machined by fast milling away of the oversize material following the contour, with a finishing cutter ( 3 ) that has a diameter-to-length ratio between 1:10 and 1:30.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a 371 of PCT/DE02/03555 filed on Sept. 21, 2002.

1. Field of the Invention

This invention relates to a method and a tool for producingheat-resistant casting molds from mold materials, especially frommolding sand containing a binder, or for producing an inner contour ofpatterns or prototypes consisting of plastic, graphite, wood materials,or other millable materials; large volumes and deep grooves are oftenproduced, especially in the machining of molds for rapid production ofprototypes from casting materials.

2. Background of the Invention

According to the prior art described in DE 195 44 573 C1, a finishingtechnique that permits a choice of standard tools is necessary for theproduction of casting molds with deep grooves. DE 195 44 573 C1 proceedsfrom the concepts that plate-like cast blanks with given, optionallyvariable, wall thicknesses are connected flush to one another insuccession with a base structure or with a previously machined blank,and are then machined upward from their downward-pointing faces withmilling tools pointing upward from the bottom, depending on the NCprogram for the layer to be machined off in each case. Productiontherefore occurs successively from plate-like layers of material. Sincerelatively thin layers of the same type always have to be machined, themilling data can be optimized for a given material. In particular, thesame milling parameters always result for given layer thicknesses forthe same material.

The use of adhesive and the additional expense for joining the layers inorder to avoid incremental errors in building up the layers aredrawbacks. The free face surface is brought to a prescribed spacingdimension with a surface miller before or after any contour machiningoperation. Support areas are necessary on the outer edge to provide thatthe next blank can be oriented and joined to the previously machinedblank exactly in the joining plane.

When using standard milling tools, the advantages of high-speedmachining are partly lost again in this way. Standard milling toolsusually include a ball end with a diameter of 6 mm and a salient lengthof 24 mm at the most. This results in a diameter-to-length ratio of from1:4 to a maximum of 1:8. Machining in individual layers necessarilyresults from this relatively small diameter-to-length ratio.

Since very high surface qualities can be produced by high-speedmachining in mold-making, subsequent fine-machining procedures can becompletely or partly omitted in many cases. Surfaces can be producedthat satisfy the required final accuracy of dimensional and shapetolerances as well as surface quality. Productive high-speed machiningtime in mold-making can be reduced considerably by higher feed rates,especially in case of complicated molds. Surface roughness with aball-head mill is determined by the given tool diameter. A small edgeradius can be produced by a toroidal cutter, but this has substantiallylarger dimensions and greater weight because of the clamping units forthe indexable inserts.

On the other hand, the cutting speeds with high-speed machining are 5–10times as high as in the conventional range, which results in raisedrequirements for the milling tools used. Milling tools for high-speedmachining are subject to high centrifugal loads. Tool imbalance, whichis of subordinate importance in conventional machining, also has to beconsidered with high circumferential speeds. Forces can be produced bytool imbalance at high speeds of rotation that are greater than theactual cutting forces. For this reason, use in high-speed machining isrestricted by structural design, especially when indexable inserts arepresent. Rising tool costs occur with increasing cutting speeds, causedby high temperatures and severe abrasive wear.

It has been found in practice that no satisfactory results can beproduced with the standard milling tools mentioned initially. This isprimarily because of the loads on the tool occurring at high frequenciesof rotation from centrifugal forces. While balanced solid tools as arule show non-critical behavior, tools with indexable inserts with highweights and large diameters especially are not a match for thecentrifugal forces in their form up to now. Against this background,suitable tools are necessary for high-speed machining.

This invention seeks to provide a method and tools with which efficiencycan be increased in the production of heat-resistant casting molds madeof mold material, especially molding sand containing binder, or in theproduction of patterns or prototypes made of plastic, graphite, woodmaterials, or other millable materials with deep grooving.

SUMMARY OF THE INVENTION

The invention provides a method for producing heat-resistant castingmolds from mold materials, especially from molding sand containingbinder, or for producing an inner contour of patterns or prototypes madeof plastic, graphite, wood materials, or other millable materials,characterized by the following steps: a) preparation of blocks of moldmaterial from mold materials, especially from molding sand containingbinder, plastic, graphite, wood materials, or other millable materialswith a dimension that typically corresponds to a mold cavity depth of upto 300 mm to 400 mm; b) machining out the inner contour of the mold byrough milling, oversized with spacing close to the inner wall of themold cavity, with a high feed rate with a milling tool operated with ahigh-frequency spindle at high speeds of rotation, in the form of aroughing tool (4) with an effective cutting diameter of 12 mm to 35 mm;c) finish-milling the mold cavity by fast millingaway of the oversizematerial following the contour, with a milling tool in the form of afinishing cutter (3) by means of a high-frequency spindle driven at highspeeds of rotation, with a smaller effective tool diameter than theroughing tool (4) and a diameter-to-length ratio between 1:10 and 1:30,preferably between 1:13 and 1:25; and d) after treatment of the surfaceof the mold material, optionally by coating and smoothing the moldcavity with mold release compound. Steps b and c preferably areperformed in a chuck and all mold elements are machined jointly.

Also provided is a milling tool in the form of a roughing tool (4) or ofa finishing cutter (3) for producing heat-resistant casting molds madeof mold material, especially of molding sand containing binder, or forproducing patterns or prototypes made of plastic, graphite, woodmaterials, or other millable materials, characterized by a rotationallysymmetrical tool shaft (7) designed and balanced as a thin-walled sleevebody, that has at the front end a cutting head (11) with one or morecutting tips (9) or blade-like inserts, and that is provided at theopposite end of the shaft with a clamping shaft (6).

Preferred milling tools can include one or more of the followingfeatures:

i. the clamping shaft (6) for clamping in the tool holder of a motorspindle is designed as a clamping shaft (6) with a smooth cylindricalshaft in the outer surface area, and in the area of the inner surface isprovided with and strengthened by a shaft body (12) inserted bypress-fitting;

ii. the shaft body (12) is provided with an adapter shaft (8) extendingbeyond the tool shaft (7) and fitting the tool holder of a motorspindle;

iii. the wall thickness of the tool shaft (7) is designed with areducing taper toward the front end (13) along its axis of rotation,with the taper of the tool shaft (7) being such that the diameter of thecutting circle of the cutting body is larger than the maximum outsidediameter of the tool shaft (7) in the area of the shaft body (12) withthe adapter shaft (8);

iv. the tool shaft (7) has a cylindrical outer surface with a constantoutside diameter, and has a non-cylindrical, conical or graduallyexpanding inner surface at least in the area between the clamping shaft(6) and the front end (13), the expansion of the sleeve being smaller inthe area of the clamping shaft (6) than in the area of the front end(13);

v. the cutting head (11) has at least one cutting tip (9), cut out ofhard metal plates, that is arranged in a positive-locking orforce-fitting manner in a slot (14) provided at the end of the toolshaft (7);

vi. in a roughing tool (4), the cutting circle diameter of the cuttingtip (9) is 12 to 40 mm, preferably 30 to 35 mm, and the tool shaft (7)has a diameter-to-length ratio between 1:3.5 and 1:11;

vii. the milling tool has an effective cutting diameter of 40 mm to 90mm, preferably between 50 mm and 80 mm, depending on the binder contentof the molding sand; and/or

viii. the diameter of the cutting circle of the cutting tip (9) is 6 mmto 12 mm, preferably 8 mm to 10 mm, and the tool shaft (7) has adiameter-to-length ratio between 1:10 and 1:30, preferably between 1:13and 1:25.

With the proposed method, casting molds can be produced with lowproduction costs and rapid production times by direct mold materialmilling. Complete machining of the casting mold in a mount is possiblewith long-salient milling tools, which makes possible rapid productionof molds, patterns, and prototypes without layerwise cementing. Theaccuracy of the overall mold can thereby be increased substantially in apreviously unknown manner. Furthermore, the cutting blades of themilling tools can be designed with small corner radii so thatcomplicated contours can be reproduced in the casting molds withgreatest accuracy. The tool shaft has a high slenderness ratio. Evencomplicated contours in small deep-lying areas of the mold, whichotherwise are inaccessible for conventional tools, can be produced withthis feature of the milling tool. Maximum cutting speeds with high feedrates are made possible by the use of modem cutting materials, and arereflected in distinct shortening of production time and improvement ofsurface quality.

The invention will be described below in further detail with referenceto an example of embodiment and to the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1—illustrates a substep of the method by machining the casting moldby rough milling,

FIG. 2—illustrates machining the casting mold with a finishing cutter,

FIG. 3—illustrates a finishing cutter with a force-fit cutting tip,

FIG. 4—illustrates a finishing cutter with a positively locked cuttingtip,

FIG. 5—illustrates a roughing cutter with force-fit cutting tip, and

FIG. 6—illustrates a roughing cutter with a positively locked cuttingtip.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a block of mold material that is to be machined as part ofa casting mold 1 with deep contours 2. The deep contours 2 are shown inFIG. 2. Casting mold 1 means heat-resistant casting molds made of a moldmaterial, especially of molding sand containing binder, or patterns orprototypes made of plastic, graphite, wood materials, or other millablematerials. Another field of use, for example, is the production ofgraphite electrodes. The term “casting mold 1” will be used below.

The casting mold 1 as a rule consists of at least two mold halves, or ofplural mold parts, which are assembled, together with the corescustomary in foundry practice, to make a complete mold. To produce acasting mold 1, a block is first made of mold material consisting of amold material or molding sand containing binder, whose dimensions as arule conform to half a casting mold. The height of the casting mold 1and thus the thickness of the block of mold material is determinedessentially by the salient length of the milling tool with which thedeep contours 2 are to be machined. The deep contours 2 are machinedwith the finishing cutter 3 illustrated in FIG. 2, which should have amaximum salient length between 300 and 400 mm. In this way, the moldcavity can have a depth of up to 400 mm, and assembling of individualcasting mold layers by cementing can be avoided. However, the methodcannot be limited to producing casting mold halves. Instead, by means ofthe method, individual blocks of mold material can be machined to thegiven dimensions and assembled into larger casting molds 1.

The machining of casting molds 1 is generally subdivided into roughingand finishing the geometry. Because of the different requirements of thetwo substeps, special procedures are advisable for roughing and forfinishing.

The purpose of roughing is the economical and rapid machining of a largevolume of material to approximate the shape for the subsequent finishingprocess. According to FIG. 1, the inner contour of the casting mold 1 isfirst machined out oversized with a milling tool in the form of aroughing tool 4, with contour-fitted clearance from the contour of theinner wall of the mold cavity. By careful roughing matching thegeometry, performed with the roughing tool 4 with a large feed and at ahigh cutting speed, the finishing process can be reduced to a minimalnecessary dimension.

The milling tools are driven at high speeds of rotation with ahigh-frequency spindle, which is not further shown. The milling toolsare held by a clamping chuck 5 that has high tool mount stiffness andhigh system damping. Press-shrunk or hot-shrunk clamping systems aresuitable for this. Clamping systems with high vibrational damping arepreferred, because deficient vibrational damping can have negativeeffects on tool life.

On the other hand, clamping chucks 5 that have a slender tool mount arepreferred for the finishing cutter 3 shown in FIG. 2, in order toprovide optimal conditions for milling in casting mold regions that aredifficult to access. Very rigid clamping systems can be used for roughmilling, to absorb high cutting and transverse forces. The clampingfunction as a rule is based on the elastic deformation of the toolmount. In contrast to this, collet chucks can be used in the same way,but they are provided with movable parts compared to forced clamp andhot-shrink technology. Symmetrical construction and optimal balancingare important for the choice of clamping means as prerequisites forhigh-precision concentricity and improvement of milling results. In thecase of collet chuck mounts, the end area of the shaft can be fasteneddirectly in the collet with the clamping shaft 6 of the tool shaft 7,while in the case of force-shrink or hot-shrink technology, theplacement of an adapter shaft 8 according to FIG. 1 extending beyond thetool shaft 7 and fitting in the tool holder is provided for.

The block of mold material in FIG. 1 is milled with a roughing tool 4with an effective cutting diameter of 12 to 40 mm. The cutting diametershould preferably be 30 to 35 mm for the production of the casting mold1 to be economical. The shaft diameter in this way is substantiallygreater than the shaft diameter for a finishing cutter 3. This can beattributed to the fact that the outside diameter of the tool shaft 7 isrelated to the bending stress on the cutting tip 9, which depends on thefree clamped length of the cutting tip 9.

Rough milling is followed by finish milling of the mold cavity with afinishing cutter 3 that is shown in FIG. 2. Rapid contour-tracingmilling off of the oversize material is done during finish milling withthe finishing cutter 3, which is clamped in one of the clamping holdersdescribed above and is driven at high speeds of rotation.

In addition, the finish-milling may include milling of molds with orwithout draft, as well as prefinishing and smoothing. For smoothing, afinishing cutter 3 with a smaller effective tool diameter than theroughing tool 4 is used, which has a diameter-to-length ratio between1:10 and 1:30, preferably between 1:13 and 1:25, and which is describedfurther below in detail. Finally, the casting mold 1 is subjected toaftertreatment, by which the surface of the mold material is sealed bycoating and the mold cavity is smoothed by applying a mold releasecompound and made ready for assembly and the casting process.

According to FIGS. 3 to 6, the milling tools necessary for high-speedmilling in the form of a roughing tool 4 or a finishing cutter 3 arecharacterized with regard to their common features by a thin-walledsleeve body that constitutes the tool shaft 7. This tool shaft 7 isbalanced and is of rotationally symmetrical design to avoid imbalance.The end of the tool shaft 7 is provided with a cutting head 11 with oneor more cutting tips 9 or blade-like inserts. At the opposite end, thetool shaft 7 has a clamping shaft 6 with a smooth cylindrical shaft onthe outer surface, which can be fastened with adequate firmness to amotor spindle by means of a collet chuck.

When using the force-shrink technique, for example, and to increaserigidity, the clamping shaft 6 is provided on the inside surface with,and strengthened by, a shaft body 12 inserted by press-fitting. Theclamping shaft 6 can thus be held directly in a collet chuck 5. To matchthe diameter of the tool shaft 7 to the given dimensions of the colletchuck 5, an adapter shaft 8 fitted to the tool holder of a motor spindlecan be provided, which extends beyond the tool shaft 7 with the shaftbody 12. This permits the use of milling tools that can be adapted to agiven collet chuck 5 and have tool shafts 7 of different outsidediameters suited to the particular machining task.

To reduce the increased weight caused by increasing the shaft diameter,the wall thickness of the tool shaft 7 according to FIG. 5 can betapered along its axis of rotation toward the front end. The taper ofthe tool shaft 7 is designed so that the diameter of the cutting circleof the cutting tip 9 is greater than the maximum outside diameter of thetool shaft 7 in the area of the shaft body 12 with the adapter shaft 8.The wall thickness can basically be tapered in by means of a conicallyextending or otherwise graduated outer surface with a constant insidediameter, which can be produced simply and accurately.

On the other hand, functional characteristics can be raised by a toolshaft 7 that has a cylindrical outer surface with constant outsidediameter and a non-cylindrical, conical or gradually expanding innersurface at least in the area between the clamping shaft 6 and the frontend 13. The widening of the sleeve in this case is smaller in the areaof the clamping shaft 6 than in the area of the front end 13. Theadvantage of the proposed method is considerably lower weight of thetool shaft 7 in the area of the cutting head 11.

The cutting head 11 has, for example, cutting tips 9 cut from hard metalplates, which are connected to the front end of the tool shaft 7 bymeans of a slot 14 by force-fitting or positive fitting. The cuttingtips 9 can be coated or made of fast-cutting steel, or can be ceramic orcement. The cutting tips 9 can be cut from a plate of material fromabout 1 mm to 3 mm thick by wire erosion. The clamping contour of thecutting tips 9 is governed by the fastening method. The economy of themethod can be further affected positively by the design and arrangementof the cutting tips 9.

A smoothing cutter 3, as shown in FIG. 3, has a cutting tip 9 with acutting circle 6 mm to 12 mm in diameter. The diameter of the cuttingcircle is preferably 8 mm to 10 mm. The corner radius 15 is 1 mm to halfthe diameter, which makes possible even castings with a small outer edgeradius. The cutting tip 9 is fastened in the cutting head 11 bypositive-fitting clamping, for example by shrinkage, the mountingexpenditure for replacing the cutting head 9 being small, given thesimple handling. The tool shaft 7 has a diameter-to-length ratio that isbetween 1:10 and 1:30, preferably between 1:13 and 1:25.

According to FIG. 4, concentricity and vibrational damping in thecutting tip 9 can be increased if the cutting tip 9 is partially mountedin a slot 14 provided diametrically in the front end 13 of the toolshaft 7. The cutting tip 9 is force-fitted in the slot 14 and firmlysecured, whereby the free salient length is reduced. The accuracy ofconcentricity can be further increased by the conical configuration ofthe tool shaft 7 shown in FIG. 5. Besides these geometry-dependentadvantages, another benefit results from a higher tool life of thecutting tip 9.

In a roughing tool 4 according to FIG. 5, the free clamping length ofthe cutting tip 9 is reduced by a tool shaft 7 that is slightly smallerthan the diameter of the cutting circle of the cutting tip 9. Dependingon the plate thickness of the cutting tip 9, the diameter of the toolshaft 7 can be 4 mm to 8 mm smaller than the diameter of the cuttingcircle of the cutting tip 9. The diameter of the cutting circle of thecutting tip can be 12 mm to 40 mm, preferably 30 mm to 35 mm. The toolshaft 7 should have a diameter-to-length ratio between 1:3.5 and 1:11.In case of a cutting tip 9 force-fitted in a slot 14 according to FIG.6, the shaft diameter chosen can be about 4 mm smaller.

1. A method for producing heat-resistant casting molds from molding sandcontaining binder, the method comprising: a) preparing blocks of moldmaterial from molding sand containing binder with a dimension thatcorresponds to a mold cavity depth of up to about 300 mm to about 400mm; b) machining out the inner contour of the block by high speed roughmilling, oversized with spacing close to the inner wall of the moldcavity, with a high feed rate with a roughing tool that is driven with ahigh-frequency spindle at high speeds of rotation and comprises abalanced, rotationally symmetrical tool shaft realized as a bendingresistant thin-walled tube or sleeve body, that has at the front end acutting head and is provided at the opposite end of the shaft with aclamping shaft, said cutting head comprising at least one cutting tip,cut out of hard metal plates, that is arranged in a positive-locking orforce-fitting manner in a slot provided at the front end of the toolshaft, the effective cutting diameter of said cutting tip being about 12to about 40 mm, and said tool shaft having a diameter-to-length ratiobetween about 1:3.5 and about 1:11, thereby providing a mold cavity; c)finish-milling the mold cavity by high speed milling, by removing theoversize material following the contour with a finishing cutter drivenat high speeds of rotation by means of the high-frequency spindle andcomprising a balanced, rotationally symmetrical tool shaft realized as abending resistant thin-walled tube or sleeve body, which has at thefront end a cutting head and is provided at the opposite end of theshaft with a clamping shaft, said cutting head comprising at least onecutting tip, cut out of hard metal plates, that is arranged in apositive-locking or force-fitting manner in a slot provided at the frontend of the tool shaft, the effective cutting diameter of said cuttingtip being smaller than that of said roughing tool and said tool shafthaving a diameter-to-length ratio between about 1:10 and about 1:30; andd) performing aftertreatment of the surface of the mold material,wherein the aftertreatment step comprises coating and smoothing the moldcavity with mold release compound.
 2. The method according to claim 1,wherein the effective cutting diameter of said cutting tip of theroughing cutter is about 30 to 35 mm.
 3. The method according to claim1, wherein the tool shaft of the finishing cutter has adiameter-to-length ratio between about 1:13 and about 1:25 wherein thetool shaft is realized as a bending resistant thin-walled tube or sleevebody.
 4. The method pursuant to claim 1, wherein steps b) and c) areperformed with the block of mold material in a chuck, with the use of aroughing tool or a finishing cutter, as the case may be, wherein theclamping shaft for clamping in the tool holder of a motor spindlecomprises a clamping shaft with a smooth cylindrical shaft in the areaof the outer surface, and in the area of the inner surface comprises astrengthening shaft body inserted by press-fitting.
 5. The methodpursuant to claim 4, wherein step b) comprises using a roughing toolwherein the clamping shaft for clamping in the tool holder of a motorspindle comprises a smooth cylindrical shaft in the area of the outersurface, and in the area of the inner surface comprises a strengtheningshaft body inserted by press-fitting, said shaft body further comprisingan adapter shaft extending beyond the tool shaft and fitting the toolholder of a motor spindle.
 6. The method pursuant to claim 5, whereinstep b) comprises using of a roughing tool wherein the wall thickness ofthe tool shaft, realized as a bending resistant thin-walled tube orsleeve body, being comprises a reducing taper toward the front end alongits axis of rotation, with the taper of the tool shaft being such thatthe diameter of the cutting circle of the cutting body is larger thanthe maximum outside diameter of the tool shaft in the area of the shaftbody with the adapter shaft.
 7. The method pursuant to claim 5, whereinstep b) comprises using a roughing tool wherein the tool shaft, realizedas a bending resistant thin-walled tube or sleeve body, comprises acylindrical outer surface with a constant outside diameter, and having anon-cylindrical, conical or gradually expanding inner surface at leastin the area between the clamping shaft and the front end, whereby theexpansion of the sleeve is smaller in the area of the clamping shaftthan in the area of the front end.
 8. A method for producingheat-resistant casting molds from molding sand containing binder, themethod comprising: a) preparing blocks of mold material from moldingsand containing binder with a dimension that corresponds to a moldcavity depth of up to about 300 mm to about 400 mm; b) machining out theinner contour of the block by high speed rough milling, oversized withspacing close to the inner wall of the mold cavity, with a high feedrate with a roughing tool that is driven with a high-frequency spindleat high speeds of rotation and comprises a balanced, rotationallysymmetrical tool shaft realized as a bending resistant thin-walled tubeor sleeve body, that has at the front end a cutting head and is providedat the opposite end of the shaft with a clamping shaft, said cuttinghead comprising at least one cutting tip, cut out of hard metal plates,that is arranged in a positive-locking or force-fitting manner in a slotprovided at the front end of the tool shaft, the effective cuttingdiameter of said cutting tip being about 40 to 90 mm, thereby providinga mold cavity; c) finish-milling the mold cavity by high speed milling,by removing the oversize material following the contour with a finishingcutter driven at high speeds of rotation by means of the high-frequencyspindle and comprising a balanced, rotationally symmetrical tool shaftrealized as a bending resistant thin-walled tube or sleeve body, whichhas at the front end a cutting head and is provided at the opposite endof the shaft with a clamping shaft, said cutting head comprising atleast one cutting tip, cut out of hard metal plates, that is arranged ina positive-locking or force-fitting manner in a slot provided at thefront end of the tool shaft, the effective cutting diameter of saidcutting tip being smaller than that of said roughing tool and said toolshaft having a diameter-to-length ratio between about 1:10 and about1:30; and d) performing aftertreatment of the surface of the moldmaterial, wherein the aftertreatment step comprises coating andsmoothing the mold cavity with mold release compound.